Project Summary/Abstract Falls and their consequences are among the major problems in the medical care of older individuals. The long-term goal of this research is to a mechanistically derived therapeutic intervention to enhance muscle power, weight-shifting capability, and lateral balance to prevent falls. When human balance is challenged, protective stepping is a vital strategy for preventing a fall during activities of daily life. Many older people at risk for falls have particular difficulties with successfully stepping sideways as a protective response to loss of balance in the lateral direction. We propose that age-related declines in lateral balance function through impaired weight transfer and protective stepping linked with falls, result from neuromuscular and biomechanical limitations in hip abductor-adductor (AB-AD) muscle power generation. Moreover, we hypothesize that these functional and neuromotor impairments can be improved with high velocity muscle resistance power training. The specigic aims are: Aim 1. To determine the age-associated changes in neuromuscular and biomechanical performance of the hip joint AB-AD musculature by evaluating the isolated maximum torque and power production and neuromuscular activation patterns. Aim 2. To determine the aging changes in neuromotor performance of the hip AB-AD musculature during the pre-step weight transfer phase of waist-pull induced side stepping and voluntary reaction time stepping. Aim 3. To establish a first line of evidence showing that hypothesized aging deficits in sidestepping caused by neuromotor impairments in hip AB-AD muscle power production may be reversible, we will determine the effects of velocity dependent muscle resistance power training (3 x/week x 10 weeks) compared with strength training on neuromuscular, biomechanical, and functional performance outcomes. Overall, the studies will identify age-related neuromotor mechanisms of abnormal hip AB-AD muscle power production that impair lateral weight transfer, balance stability, and mobility function. Establishing a first line of support for the superiority of velocity dependent power training over strength training on muscle performance and protective balance and functional mobility outcomes, will lead to a future comparative intervention trial to enhance these functions and prevent falls in older adults.